The present invention relates to a process for producing cyanuric acid by decomposing urea. More particularly, the invention provides extremely advantageous and economical process in an industrial process for producing cyanuric acid by subjecting urea to heat treatment by the use of an external heating type reaction vessel furnished with agitator such as a kneader or a paddle mill.
With regard to the process for producing cyanuric acid from urea, various procedures have already been known.
Hitherto it has been well known that, when urea is heated, decomposition reaction progresses through the change of states in order of solid state.fwdarw.transparent low viscosity liquid state.fwdarw.white turbid state.fwdarw.paste state.fwdarw.solid state until completion of reaction. Under the ordinary heat treatment process the reaction products adhere to the heat transfer surface and the agitator blades and take block form, thereby providing difficulty to continuous operation, and moreover, leading to lowering of yield. For these reasons, various methods to solve these problems have been proposed.
For example, U.S. Pat. No. 2,943,088 discloses a method for continuous production of crude cyanuric acid by feeding a part of crude cyanuric acid discharged from a kiln to a pug mill, adding a required amount of urea, and mixing, followed by recycling it to the kiln to produce crude cyanuric acid continuously.
Besides the above, Japanese patent publication No. 256,76/74 discloses a continuous process for producing crude cyanuric acid by pulverizing the crude cyanuric acid calcined at low temperature discharged from No. 1 pug mill, adding thereto a designed amount of urea, and mixing to form a low-temperature calcined crude cyanuric acid, pulverizing it and recycling a part of it to No. 1 pug mill, with the remainder fed to No. 2 pug mill to produce crude cyanuric acid continuously.
In these methods, in either case, supply ratio of urea to crude cyanuric acid is placed under restriction. Unless the crude cyanuric acid of about three fold amount or more to the urea in ratio by weight is recycled, smooth continuous operation cannot be maintained.
In other words, according to the present inventors' experience, when the amount of supply of the urea to the recycled amount of crude cyanuric acid exceeds about 1/3 in ratio by weight, adhesion of contents to the inner wall of the reactor and the agitator progress and generation of block products is caused, thus making it difficult to carry out continuous operation.
Accordingly, in order to secure smooth operation through these means, it is necessary to cycle three or more parts by weight of crude cyanuric acid to 1 or less part by weight of material urea to be supplied. This makes it necessary to enlarge remarkably the size of the reaction vessel. It accompanies degradation in production efficiency and loss of energy, etc. which cannot be accepted as a satisfactory condition.